Users: Adjust [45708] to make sure `wp_update_user()` does not issue …

…a `WP_Error` if a single site was previously set up as Multisite and there's still a `spam` field in the user table.
Add a unit test.
Props azaozz, SergeyBiryukov.
Fixes #45747.
git-svn-id: https://develop.svn.wordpress.org/trunk@45874 602fd350-edb4-49c9-b593-d223f7449a82